Corrosion behavior of plasma-sprayed Al x CrCoFeNi high-entropy alloy coatings on austenitic steel

Abstract

High-entropy alloy (HEA) coatings are emerging as promising surface engineering materials for enhancing corrosion resistance in aggressive chloride-containing environments; however, a clear correlation between aluminium (Al) content, microstructural evolution, and electrochemical performance in plasma-sprayed Al_xCrCoFeNi HEA coating remains insufficiently understood. In the current study, plasma-sprayed Al_xCrCoFeNi HEA coatings with varying Al fractions (x = 0, 0.5, 1) content were deposited on the SS-304 substrates. Microstructural characterization revealed that increasing the Al molar fraction content improved coating integrity. Porosity decreased from ∼7% (x = 0) to ∼2% (x = 1), while microhardness increased from 420 ± 36 HV (x = 0) to 530 ± 15 HV (x = 1). XRD analysis demonstrated enhanced crystallinity with Al addition, rising from ∼87% (x = 0) to ∼90.6% (x = 1). Electrochemical testing in 3.5 wt.% NaCl solution of HEA coatings revealed a strong dependence of corrosion resistance on the Al molar fraction content. The equimolar Al₁CrCoFeNi coating exhibited the noble corrosion potential (–0.379 V), lowest corrosion current density (3.0 × 10⁻⁷ A/cm²), and minimum corrosion rate (0.0040 mm/year) amongst all three HEA coatings. Nyquist and Bode plots also confirmed superior charge transfer resistance and passive film stability for the equimolar coating. Post-corrosion SEM images validated the formation of dense Al₂O₃-rich oxide layers that suppressed localized corrosion in the A₁ composition. The results establish a direct correlation between Al-induced phase evolution, densification, passive film formation, and enhanced corrosion performance, identifying equimolar A₁ coating is a highly effective composition for durable protective coatings in marine and chloride-rich environments.